Hydrodynamics of circumbinary accretion: Angular momentum transfer and binary orbital evolution

TL;DR

This study uses 2D hydrodynamical simulations to analyze how circumbinary accretion influences angular momentum transfer and orbital evolution, revealing that binaries tend to gain angular momentum and expand over time.

Contribution

It provides the first detailed simulation-based analysis of angular momentum transfer and orbital evolution in eccentric equal-mass binaries during circumbinary accretion.

Findings

Binary gains angular momentum from accretion.

Binary orbital radius increases over time.

Angular momentum transfer balances the flux across the disk.

Abstract

We carry out 2D viscous hydrodynamical simulations of circumbinary accretion using the AREPO code. We self-consistently compute the accretion flow over a wide range of spatial scales, from the circumbinary disk (CBD) far from the central binary, through accretion streamers, to the disks around individual binary components, resolving the flow down to 2% of the binary separation. We focus on equal-mass binaries with arbitrary eccentricities. We evolve the flow over long (viscous) timescales until a quasi-steady is reached, in which the mass supply rate at large distances $\dot{M}_0$ (assumed constant) equals the time-averaged mass transfer rate across the disk and the total mass accretion rate onto the binary components. This quasi-steady state allows us to compute the secular angular momentum transfer rate onto the binary, $\langle\dot{J}_b\rangle$, and the resulting orbital evolution. Through direct computation of the gravitational and accretion torques on the binary, we find that $\langle\dot{J}_b\rangle$ is consistently positive (i.e., the binary gains angular momentum), with $l_0\equiv\langle\dot{J}_b\rangle/\dot M_0$ in the range of $(0.4-0.8)a_b^2\Omega_b$, depending on the binary eccentricity (where $a_b,~\Omega_b$ are the binary semi-major axis and angular frequency); we also find that this $\langle\dot{J}_b\rangle$ is equal to the net angular momentum current across the CBD, indicating that global angular momentum balance is achieved in our simulations. We compute the time-averaged rate of change of the binary orbital energy for eccentric binaries, and thus obtain the secular rates $\langle\dot a_b\rangle$ and $\langle \dot{e}_b\rangle$. In all cases, $\langle\dot{a}_b\rangle$ is positive, i.e., the binary expands while accreting. We discuss the implications of our results for the merger of supermassive binary black holes and for the formation of close stellar binaries.